The present invention relates to corrosion monitoring devices and more particularly to a self contained, disposable corrosion monitor for a fluid containing system to indicate a preselected loss of a predetermined metal on an inner wall surface of the system due to the corrosive action of a fluid in the fluid containing system.
Failures of metal pipes, tanks and fluid enclosures due to the deterioration of their inner wall surfaces are all too common occurrences; currently causing over 10 billion dollars annually in replacement costs alone. While some corrosion induced metal failures may create a great and unexpected financial loss due to replacement cost, down time and water damage, the more severe failures of high temperature and high pressure pipes and vessels may result in explosion, extensive physical damage, severe injury and human casualty. Effective corrosion monitoring is therefore greatly desired from such a production, health, environmental, process reliability, economic and liability viewpoint.
Corrosion of metal surfaces is a continuous and generally non-stoppable electrochemical process which is well known and documented. Given sufficient time, metal failures are inevitable where fluids and metal meet and interact, and where at best, the negative physical effects of corrosion can only be minimized, not eliminated.
Due to the complex interaction of chemical, electrical and mechanical influences which determine the degree of activity of a corrosion environment, and the fact that such corrosion activity often widely varies at different locations within the same fluid environment, there is the need to monitor as many individual locations in a given piping system, tank, pressure vessel or reactor vessel as possible. The inability to monitor the corrosion activity at multiple locations within a piping system is frequently a cause of failure, since a single monitoring point, not representative of the corrosion in the entire piping system, may produce an erroneous prediction of surface life and surface failure.
Chemical and electrical corrosion inhibitors and other substances exist to help reduce corrosion and are often relied upon exclusively to safeguard piping and other metal components. The use of such corrosion inhibitors does not preclude the need for corrosion monitoring, however, since actual results of anti-corrosion agents vary widely, thereby making it necessary to regularly verify their efficiency and proper application.
A wide variation of corrosion monitors and procedures exist in the literature and in current use with the purpose of measuring the corrosive nature of a fluid or fluid stream against a specific metal surface. These devices may be mechanical, electrical or chemical in design and measuring instantaneous, average or accrued corrosion, and typically produce a corrosion rate measurement defined in mils per year (MPY).
Various types of currently available electronic instrumentation provide instantaneous, real time detail of the corrosion rate of MPY through the use of the insertion probes measuring changes in voltage, oxidation potential or other electro-chemical characteristics associated with a corrosion condition. Such devices fail to provide accumulated or total loss of metal in inches, require remote wiring to a main display instrument location, are sophisticated in design and, therefore, expensive to purchase and require regular maintenance, recalibration and supervision. Such devices, therefore, find limited actual field use, provide unsatisfactory corrosion monitoring coverage due to the limited number of sensors or probes installed and provide little benefit in all but the most critical of applications.
The most commonly used mechanical corrosion measuring device, termed a "corrosion coupon", consists of a thin flat bar of a specific composition metal (typically steel, brass, stainless steel or copper) having the approximate dimension of 1.75 in..times.0.75 in..times.0.125 in. One or more corrosion coupons are typically inserted in a by-pass flow assembly located external to the fluid system. This flow assembly routes a small volume of the fluid to be evaluated across the metal corrosion coupon surface. Under limited physical conditions permitting such use, corrosion coupons may be also inserted directly into a pipe or pressure vessel. The possibility of losing the coupon in the tank or circulating system generally precludes such use however.
Corrosion coupons are precisely preweighed in a laboratory prior to use, left within the flow assembly pipe or pressure vessel to naturally deteriorate over a given period of time (typically 60 to 180 days), and then removed for a follow-up laboratory analysis. Weight loss is measured, and a calculation made extrapolating the overall weight loss into a determination of MPY.
Ultrasonic testing is well recognized for providing extremely accurate remaining wall thickness measurements for any metal structure, but typically serves as a survey or an instantaneous measuring tool rather than a long term monitoring device. It has a disadvantage of being a temporary measurement instrument, is expensive, requires an experienced operator and careful analysis and manipulation of the resulting data, as well as periodic access to the exact same area of the pipe surface for reevaluation.
Additionally, corrosion monitoring techniques can include the boring from the outside metal surface a plurality of holes which will be corroded through prior to that estimated to be the final rupture point. When the remaining wall thickness of the bores has been breeched by corrosion, such holes are at various locations throughout the test subject provide a "telltale" indication in a form of a small and controllable leak thereby signaling the need for repair or replacement prior to a greater, and potentially catastrophic failure. Once a telltale hole is penetrated, it must be repaired. Future telltale holes cannot be located at previous sites.
An alternative method of measurement of corrosion has been to bore completely through the metal wall in order to provide access for a gauging or inspection device and temporarily plugging the test opening between test cycles.
In a more complicated arrangement a plurality of bores are set for preselected depths from the outer surface of the system in the actual metal of a system past a point at which failure is expected to occur (borings are made closer to the interior of the test surface). Radioactive tracer elements are placed in the bores and sealed in place. Corrosion of the interior metal surface, once it penetrates through to the bore, releases the tracer element into the fluid flow for electronic detection at a central location. Such testing would be environmentally difficult to pursue today.
Another system relates to a device comprising both a pipe and a telltale plug, whereby the fluid corrodes through the pipe material to reach the plug and produce a visual leak. Telltale plug serves as a conduit for the fluid only. It requires modification to the piping system by boring, or taping into the pipe, exposing the pipe system to an additional threat. It cannot be used in thin wall applications and exposes individuals to the hazard of fluid leakage and contamination.
The prior art devices known use the corrosive break through of the piping system itself rather than a surface of the plug. The composition of the device determines the type of metal corrosion to monitor. Also the prior art intended to monitor the increasing corrosivity of a fluid before the point it will damage an internal combustion engine or the like, rather than monitor the actual amount of metal loss.